Can you save electricity by dimming both morning and afternoon?

Phalaenopsis growers sometimes use artificial lighting for 16 hours per day, totalling 8 to 10 mol/m2/day, especially in the winter. For a crop with a cultivation period of 40 to 50 weeks, lighting is therefore a major expense. Measurements show that the correct timing of artificial lighting is more important than the total light sum. Additional research should show whether growers can indeed save around 30% on the lighting hours. Before making such a big change in mindset and operation, growers want absolute certainty.

Since 2012 light specialists Sander Hogewoning and Govert Trouwborst of research centre Plant Lighting, of Bunnik the Netherlands, have been researching the ideal lighting for phalaenopsis. They’ve been doing that with colleagues from Plant Dynamics. It’s rewarding work because very little scientific research has been carried out on this subject. As a result there’s plenty to discover. The Dutch program Kas als Energiebron (Greenhouse as Energy Source) – whose goal is to save energy – was the most important investor in the trials. The crop alliance for pot orchids, of the growers association, LTO Glaskracht Nederland, also contributes and is actively involved in the research.

How much lighting makes sense?

Hogewoning outlines the situation from the beginning. Phalaenopsis is a CAM-plant, just like pineapple, agave and cactus. This means that photosynthesis is different to ordinary C3-plants. CAM-plants are slow growers who only open their stomata in the afternoon. They take up CO2 during the afternoon and night and store it in cells in the form of malate. Storage space is thereby limited. Photosynthesis takes place during the day because light is needed for this. When the malate is all used up photosynthesis for the most part stops.
“Growers now use a lot of light even if the plant isn’t using the light. How much lighting is actually useful for a CAM-plant? And when?” asks the researcher.

Long night unfavourable

In a study in 2014 the researchers showed that a light sum of 6.5 mol/m2/day is enough to fulfil the maximum storage for malate, under the condition that the leaves remain fairly horizontal. More lighting does not yield any more photosynthesis. In addition, the trials showed that allowing in more light during the later phase of cultivation compared with the early phase did not lead to any extra photosynthesis. Another interesting conclusion: Lighting fully in the early morning and afternoon doesn't make sense: During those hours electron transport in the leaves is low.
Trouwborst: “Nevertheless, it is also unwise to provide no light during these inefficient hours. Continuously lighting for 16 hours yielded better results than maintaining a long night and providing 12 hours of light. Growers in practise experience the same. Therefore we wanted to look for the best lighting recipe.”

Step-by-step or low intensity?

This was the focus of the research in 2015. Hogewoning explains his goal. “The aim is to save energy by using less lighting during the hours when the plant hardly uses the light. The question is how far can you dim in the morning and/or the afternoon without loss of production?"
The researchers trialled the variety Sacramento over two sessions. One trial ran from mid March to end May, the second ran from the start of June to the end of July 2015. They started with plants that had already been through the cultivation and cooling phases. They divided them over eight climate units, each of 25 plants. The units simulated daylight similar to that in a winter situation.
Each unit received its own light recipe using SON-T-lighting for about eight weeks. One of these was the control that received lighting similar to that normally used in practise. This was 7 mol/m2/day dosed over 16 hours. Four treatments were dynamic and based on previous research. “For these treatments we maintained a day length of 16 hours. During the inefficient hours in the morning and evening we dimmed the SON-T light to a greater or lesser extent. We switched them on and off in different ways. This saved between 8 and 33 per cent electricity. In addition, two treatments had a day length of 11.5 hours.”

Dimming the light works

The results were very positive. They confirmed the hypothesis that dimming at certain hours is possible and agreed with the results of previous research. The total CO2 uptake for each of the four dynamic treatments was the same as the control treatment of 7 mol/m2/day over 16 hours - even the same as the treatment that saved more than 30% on energy.
Hogewoning: “The savings are substantial; in practise that can amount to hundreds of thousands of euros, not only on electricity costs, but also because growers can replace the bulbs less often.” Trouwborst adds: “Therefore it’s not about achieving the correct light sum but the right timing. You have to use the right light intensity at the right moment.”
Hogewoning points out another important result. “A long night came out the worst, which we also discovered in 2014. When we continuously lit for 11.5 hours, 7 mol/m2/day, the CO2-uptake by the plant is less than in the dimming treatment with a 16 hour day length and a light sum of 5.2 mol/m2/day.”

Questions from commercial growers

These are spectacular results, but will phalaenopsis growers dare to dim the SON-T-lights early in the morning and afternoon? Trouwborst: “We recently presented these results to the supervisory commission and the crop alliance for pot orchids. Their first question was: ‘Does anyone do this in practise already?’ With millions of euros of plant material at stake growers won’t so easily alter their lighting methods. That’s why it is important to slowly scale up this research. The crop alliance is following this very closely.”
Growers, of course, want to know if a good uptake of CO2 translates into plant quality and number of buds. The researchers have also measured that but Hogewoning points out: “The number of plants was too small to draw conclusions. However we do see that the number of flowers is in line with the CO2-uptake. The treatment with the least uptake – a long night, 5.2 mol/m2/day – clearly led to fewer flowers.”

Repeat trial

Because they want to see these positive results confirmed on a larger scale, the researchers are running a repeat trial with more plants. For this they invested in new climate units of over 2 m2, in addition to their existing smaller units. “In these new units we can even let the sun slowly rise and set. All year around we can mimic the light situation in the winter, without being dependent on the weather conditions,” explains Hogewoning.
The trial was due to run until the end of May 2016. In consultation with the growers the researchers ran the two most successful treatments as well as the control treatment, which is similar to that currently used in practise. Hogewoning: “The crop alliance is very curious about the results. They are waiting with baited breath. In the meantime growers who have questions can discuss it within the group.”

Summary

Commercial phalaenopsis growers use a lot of artificial lighting. Research indicates that the timing of the lighting is more important than the total light sum. It seems that growers can dim the lighting early in the morning and in the afternoon without any negative effects. If these initial results are confirmed it appears that at least 30% can be saved on electricity.